The invention relates to multilayer interference pigments with plateletlike titanium dioxide as substrate.
Multilayer pigments of low transparency are known. The metal oxide layers are prepared either in a wet process, by precipitating the metal oxide hydrates from a metal salt solution onto a carrier material, or by vapour deposition or sputtering under reduced pressure. In general, the vapour deposition processes are too complex and costly for mass production of pigments. Thus U.S. Pat. No. 4,434,010 describes a multilayer interference pigment consisting of a central layer of a reflecting material (aluminium) and alternating layers of two transparent, dielectric materials of high and low refractive index, for example titanium dioxide and silicon dioxide, either side of the central aluminium layer. This pigment is employed for the printing of securities.
JP H7-759 (Kokoku) describes a multilayer interference pigment with a metallic lustre. It consists of a substrate coated with alternating layers of titanium dioxide and silicon dioxide. The substrate is formed from flakes of aluminium, gold or silver or from platelets of mica and glass which are coated with metals. Accordingly, it is a typical metal-effect pigment. This pigment is of high opacity. For applications where a high level of transparency of the pigmented material is required, for example for agricultural films, the pigment is unsuitable. Furthermore, it has the disadvantage that the depth effect typical of interference pigments is not produced since, owing to the total reflection of the light at the metal layer which forms the core, a number of pigment particles are unable to enter into interaction. The interference effect therefore remains limited to the coats located on the metal layer.
Mica is the substrate employed most frequently for the production of interference pigments.
Mica pigments are used widely in the printing and coating industries, in cosmetics and in polymer processing. They are distinguished by interference colours and a high lustre. For the formation of extremely thin coats, however, mica pigments are not suitable, since the mica itself, as a substrate for the metal oxide coats of the pigment, has a thickness of from 200 to 1200 nm. A further disadvantage is that the thickness of the mica platelets in some cases varies markedly about a mean value. Moreover, mica is a naturally occurring mineral which is contaminated by foreign ions. Furthermore, technically highly complex and time-consuming processing steps are required including, in particular, grinding and classifying.
Pearl lustre pigments based on thick mica platelets and coated with metal oxides have, owing to the thickness of the edge, a marked scatter fraction, especially in the case of relatively fine particle-size distributions below 20 xcexcm.
As a substitute for mica it has been proposed to use thin glass flakes which are obtained by rolling of a glass melt with subsequent grinding. Indeed, interference pigments based on such materials exhibit colour effects superior to those of conventional, mica-based pigments. Disadvantageous, however, is that the glass flakes have a very large mean thickness of about 10-15 xcexcm and a very broad thickness distribution (typically between 4 and 20 xcexcm), whereas the thickness of interference pigments is typically not more than 3 xcexcm.
EP 0,384,596 describes a process in which hydrated alkali metal silicate is subjected at temperatures of 480-500xc2x0 C. to the action of an air jet, forming bubbles with thin walls; the bubbles are subsequently comminuted to give plateletlike alkali metal silicate substrates with a thickness of less than 3 xcexcm. However, the process is complex and the thickness distribution of the resulting platelets is relatively broad.
DE 11 36 042 describes a continuous belt method of preparing plateletlike or glitterlike oxides or oxide hydrates of metals of groups IV and V and of the iron group of the Periodic Table. In this method, a release layer comprising, for example, a silicone coating is first of all applied, if desired, to a continuous belt in order to facilitate the subsequent detachment of the metal oxide layer. Then a liquid film is applied which comprises a solution of a hydrolysable compound of the metal which is to be converted into the desired oxide, and the film is dried and subsequently detached using a vibration device. The coat thickness of the platelets obtained is given as being from 0.2 to 2 xcexcm, although no concrete examples of this are cited.
EP 0 240 952 and EP 0 236 952 propose a continuous belt method of preparing different plateletlike materials, including silicon dioxide, aluminium oxide and titanium dioxide. In this method, a thin liquid film of defined thickness of a precursor of the plateletlike material is applied, via a roller system, to a smooth belt; the film is dried and detached from the belt, forming plateletlike particles. The particles are subsequently, if desired, calcined, ground and classified.
The thickness of the platelets obtained in accordance with the method described in EP 0 240 952 is relatively well defined, since the film is applied very uniformly, for example to the continuous belt via a roller system. The layer thickness of the platelets is given in the examples as being from 0.3 to 3.0 xcexcm. According to Example 1, a first roller is wetted with the precursor used by immersing this roller partially into a stock container which is filled with the precursor. The film is transferred from this roller to a second, co-rotating roller which is in very close contact with the first roller. Finally, the film is rolled off from the second roller onto the continuous belt.
Disadvantages, however, are the use of very expensive precursor materials and, in particular, the increased requirements in terms of workplace safety which must be applied when organometallic compounds are used. The complete chemical conversion of the precursor into the desired coating material requires, in general, high heating of the film and of the belt material. In addition to the considerable thermal stress which this places on the belt material, the high energy consumption and the restriction on the process speed are also highly disadvantageous for the economy of the method.
WO 93/08 237 describes plateletlike pigments consisting of a plateletlike matrix comprising silicon dioxide, which may contain soluble or insoluble colourants and which is coated with one or more reflecting layers of metal oxides or metals. The plateletlike matrix is prepared by solidification and hydrolysis of water glass in a continuous belt.
DE 12 73 098 describes the preparation of a mother-of-pearl pigment by vapour deposition of ZnS, MgF2, ZnO, CaF2 and TiO2 films on to a continuous belt. This process however, like the process described in U.S. Pat. No. 4,879,140 in which plateletlike pigments with Si and SiO2 coats are obtained by plasma deposition from SiH4 and SiCl4, is associated with very high expenditure on apparatus.
The object of the invention is to provide an essentially transparent interference pigment having strong interference colours and/or a strong angular dependency of the interference colours. Furthermore, the object of the invention is to provide a pigment which consists only of optically functional layers and is therefore extremely thin.
This object is achieved in accordance with the invention by a multilayer interference pigment consisting of plateletlike titanium dioxide as carrier material, coated with alternating layers of metal oxides of low and high refractive index, the difference in the refractive indices being at least 0.1, which is obtainable by solidification and hydrolysis of an aqueous solution of a thermally hydrolysable titanium compound on a continuous belt, detachment of the resulting coat, coating of the resulting titanium dioxide platelets, with or without drying in between, by a wet method with, alternately, a metal oxide hydrate of high refractive index and a metal oxide hydrate of low refractive index by hydrolysis of the corresponding, water-soluble metal compounds, separation, drying and, if desired, calcining of the material obtained.
This object is additionally achieved in accordance with the invention by a process for preparing the novel pigment, in which
an aqueous solution of a thermally hydrolysable titanium compound is applied as a thin film to a continuous belt,
the liquid film is solidified by drying, during the course of which the titanium dioxide is developed from the solution by means of a chemical reaction,
the resulting layer is subsequently detached from the belt and washed,
the titanium dioxide platelets obtained, with or without drying in between, are suspended in water and coated with, alternately, a metal oxide hydrate of high refractive index and a metal oxide hydrate of low refractive index, by addition and hydrolysis of the corresponding, water-soluble metal compounds, and
the coated titanium dioxide platelets are separated out from the aqueous suspension, dried and, if desired, calcined.
The invention additionally relates to the use of the novel pigments for pigmenting paints, printing inks, plastics, cosmetics and glazes for ceramics and glass and for producing agricultural films.
For this purpose they can be employed as mixtures with commercially available pigments, for example inorganic and organic absorption pigments, metal-effect pigments and LCP pigments.
The novel pigments are based on plateletlike titanium dioxide. These platelets have a thickness of between 10 nm and 500 nm, preferably between 40 and 150 nm. The extent in the two other dimensions is between 2 and 200 xcexcm and, in particular, between 5 and 50 xcexcm.
The metal oxide of high refractive index can be an oxide or mixture of oxides with or without absorbent properties, for example TiO2, ZrO2, Fe2O3, Fe3O4, Cr2O3 or ZnO, or a compound of high refractive index such as, for example, iron titanates, iron oxide hydrates and titanium suboxides, or mixtures and/or mixed phases of these compounds with one another or with other metal oxides.
The metal oxide of low refractive index is SiO2, Al2O3, ALOOH, B2O3 or a mixture thereof and can likewise have absorbent or nonabsorbent properties. If desired, the oxide layer of low refractive index may contain alkali metal oxides and alkaline earth metal oxides as constituents.
The thickness of the layers of the metal oxides of high and low refractive index is critical for the optical properties of the pigment. Since a product with strong interference colours is desired, the thicknesses of the layers must be adjusted relative to one another. If n is the refractive index of a layer and d its thickness, the colour which appears in a thin layer is the product of n and d, i.e. the optical thickness. The colours of such a film, as produced with normal incidence of light in reflected light, result from an intensification of the light of wavelength xcex=(4/2Nxe2x88x921.nd and by attenuation of light of wavelength xcex=(2/N.nd, where N is a positive integer. The variation in colour which takes place as the thickness of the film increases results from the intensification or, respectively, attenuation of particular wavelengths of the light by interference. For example, a 115 nm thick film of titanium dioxide of refractive index 1.94 has an optical thickness of 115xc3x971.94=223 nm, and light of wavelength 2xc3x97223 nm=446 nm (blue) is attenuated in the course of reflection, with the result that the reflected light is yellow. In the case of multilayer pigments, the interference colour is determined by the intensification of specific wavelengths and, if two or more layers in a multilayer pigment possess the same optical thickness, the colour of the reflected light becomes more intense and full as the number of layers increases. Moreover, by a suitable choice of the layer thicknesses it is possible to achieve a particularly marked variation of colour in dependency on the viewing angle. A pronounced colour flop develops, which may be desirable for the pigments according to the invention. The thickness of the individual metal oxide layers, independently of their refractive index, is therefore from 20 to 500 nm, preferably from 50 to 300 nm.
The number and thickness of the layers is dependent on the desired effect. Normally, the desired effects are achieved if the 5-layer system TiO2/SiO2/TiO2/SiO2/TiO2 is built up and if the thicknesses of the individual layers are matched optically to one another. When using optically relatively thin TiO2 layers (layer thickness  less than 100 nm) it is possible, for example, to produce interference pigments with a blue colour which, with a substantially smaller TiO2 content, are stronger in colour and more transparent than pure TiO2-mica pigments. The saving in terms of TiO2 is up to 80% by weight. By means of the precipitation of thick SiO2 layers (layer thickness  greater than 100 nm), pigments having a strongly pronounced angular dependency of the interference colour are obtained.
By precipitating further SiO2 and TiO2 layers it is also possible to obtain higher systems, the number of layers then being limited by the economics of the pigment.
Since, in contrast to mica, plateletlike titanium dioxide as substrate is an optically functional layer, covering the substrate with, for example, two layers of the abovementioned structure gives an interference system comprising 5 thin layers of sharply defined thicknesses. The reflection or transmission spectrum of such a pigment exhibits finer and more precisely matchable structures than the spectrum of a corresponding pigment based on a substrate with a broad thickness distribution, such as mica.
Even with extremely thin TiO2 layers (layer thickness: 40 nm), these pigments exhibit strong interference colours. The angular dependency of the interference colour is also particularly pronounced. This extreme colour flop is not observed with conventional metal oxide-mica pigments.
The novel pigments are prepared in a two-stage process. In the first stage, plateletlike titanium dioxide particles are prepared with the aid of a continuous belt.